Electrophoresis, an electrokinetic phenomenon, is used mainly in analytical and clinical chemistry for analyzing mixtures of substances, where one determines the different mobility of ions in an electric field. In addition, electrophoresis in porous media--such as polyacrylamide or agarose gels--has been developed to an important analytical and also preparative method in biochemistry and medicine, e.g., for separating complex mixtures of nucleic acids or proteins into single components. After the application of an electrical field, different molecules move different distances into the gel (which includes all the substances suitable for this purpose) dependent on their mobility and are, thereby, separated in space. By specific staining of these molecules, their distribution in the mixture can be visualized and analyzed. A disadvantage is that the gel has to be removed from the electrophoresis apparatus and that it can be used only once. To achieve a satisfactory separation of many components--for example in the sequence analysis of DNA including up to a few hundred molecules of different lengths--gels of up to one meter length are currently used; further, a very high voltage has to be applied to obtain a large voltage gradient, which allows a separation within reasonable time.
In preparative electrophoresis, a stationary collection chamber is usually provided at the end of the gel, into which the molecules move. This chamber is emptied from time to time and is then filled again; in this way, a stepwise separation of components of the mixture can be achieved.